WASHINGTON (AP) _ A 20-year-old drug available without prescription in Mexico and Japan puts up a strong fight against the AIDS virus in laboratory tests, but a National Cancer Institute researcher says the finding ''is not a breakthrough.''

The drug, called dextran sulfate, was found to prevent the AIDS virus from infecting and killing the body's T-cell lymphocytes, the main target of the virus, researchers say in a Science article published today.

But Dr. Samuel Broder of NCI, one of the co-authors of the research paper, said in an interview that much still remains to be learned about the drug before it could possibly be used to treat AIDS.

''This is not a breakthrough,'' said Broder. He said that though the drug ''is a potent agent'' against the AIDS virus in a test tube, it is not clear that the drug can be given to patients in strong enough doses to affect the course of the disease.

''We found in the test tube that this agent is a very a powerful inhibitor of the HIV virus,'' said Dr. Flossie Wong-Staal, a NCI researcher and co- author of a study on dextran sulfate. HIV, for human immunodeficiency virus, is the virus that causes acquired immune deficiency syndrome.

Dr. Wong-Staal said the drug has been used for more than two decades in Japan and elsewhere as an anticoagulant and has demonstrated that it has no significant toxicity.

Broder said dextran sulfate is only one of a large family of potential anti-virals that must first undergo carefully controlled trials.

Dr. Robert Gallo, an NCI researcher and co-discoverer of the AIDS virus, said dextran sulfate should be studied further.

''I'm impressed by the results I've seen in vitro and it merits trials (in patients),'' he said. ''The question is will it be able to be taken by patients for long periods of time.''

A spokesman at NCI said dextran sulfate now is in clinical trials at San Francisco General Hospital and results from the tests are expected to be announced at an international AIDS meeting in Stockholm in June.

Dr. Wong-Staal said the drug prevents the HIV virus from invading the T- cell lymphocytes, a type of immunity cell that the AIDS virus normally attacks.

Usually the HIV virus attacks the T-cells by first attaching to a receptor, a type of protein, on the cell's outer layer or envelope. In the test tube studies, dextran sulfate was found effective in preventing the virus from binding to the cell envelope, thus preventing it from invading and killing the cell.

Untreated, HIV will change the genetic pattern of the T-cell and force it to start making copies of the virus. But this cannot take place if the virus cannot invade the cell.

The drug was first formulated by the Uneo Fine Chemical Industry of Osaka, Japan. It was used for decades to treat blood clots, but has not been approved for that use in the United States by the Food and Drug Administration. The drug can be purchased routinely over the counter in Mexico and in Japan, however.

Last May, the FDA approved the drug for use in limited clinical trials in San Francisco, and Uneo now holds a patent for its use against AIDS.

AIDS is a contagious disease that attacks the body's immune system, rendering it incapable of resisting other diseases and infections. The virus most often is spread through close contact with blood, blood products or semen from infected persons.

The chief victims of AIDS in the United States have been homosexual men and intravenous drug users, although a small percentage of cases are attributed to transfusions of contaminated blood, heterosexual contact and spread from infected pregnant women to their offspring.

As of April 21, AIDS had been diagnosed in about 59,500 Americans, of whom more than half, or about 33,000 have died since June 1981, according to the federal Centers for Disease Control.

No one is known to have recovered from AIDS.